It is a well-established fact that the most important gas turbine engine parameter that can benefit fuel and capital conservation goals is the turbine inlet temperature. In the present state of the art, this temperature is limited because both the blades and the rotor disk of a turbine rotor are metallic and cannot withstand the gas temperatures above certain maximum values.
Ceramic materials are currently under investigation for use in making turbine blades. The major roll of ceramics is their potential capability to operate at inlet temperatures and in corrosive environments that far exceed the capability of any uncooled alloy system. The use of cooling schemes needed for metals, in one way or the other, must use part of the available energy to remove heat from high temperature locations. This results directly in efficiency penalties. The ultimate goal is to make the best use of ceramics in an efficient combined cycle (gas turbine/steam turbine plant) that uses coal derived fuels. To use ceramics in turbine blade designs, a specific rotor configuration is one having a low alloy steel disk, an intermediate superalloy attachment piece, and a ceramic blade. This approach allows the best use of ceramics as well as the design flexibility needed for rugged industrial gas turbines.
The root attachment section of a gas turbine first stage rotating ceramic blade has been identified as the critical area for the development of stress problems. Centrifugal forces and thermal conditions combine to generate high loads which must be transferred at the surface of contact between the root of the blade and the surface defining the groove of the intermediate piece which attaches the blade to the disk.
The rigidity and lack of ductility of the ceramic root of a ceramic blade combine to create high direct local compressive stresses and high secondary tensile stresses. These arise from the misfit between the bearing surface of the root and the bearing surface defining the groove. The tensile stresses are augmented by friction between the two bearing surfaces that results when loads are applied, i.e., centrifugal force field and differential thermal expansion of the components. Because of these problems, a need has arisen for an improved ceramic turbine blade which is designed and constructed to transfer the load substantially uniformly between the blade root and the disk of the turbine so as to permit the turbine to be operated at high inlet gas temperatures.